net/http: reduce allocs in CrossOriginProtection.Check

Rather than repeatedly creating error values on
CrossOriginProtection.Check's unhappy paths, return non-exported and
effectively constant error variables.

For #73626.

Change-Id: Ibaa036c29417071b3601b8d200ab0902359d1bb9
GitHub-Last-Rev: e704d63
GitHub-Pull-Request: #74251
Reviewed-on: https://go-review.googlesource.com/c/go/+/681178
Reviewed-by: Sean Liao <[email protected]>
Reviewed-by: qiu laidongfeng2 <[email protected]>
Reviewed-by: Junyang Shao <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: jub0bs

src/net/http/csrf.go

@@ -136,7 +136,7 @@ func (c *CrossOriginProtection) Check(req *Request) error {
 		if c.isRequestExempt(req) {
 			return nil
 		}
-		return errors.New("cross-origin request detected from Sec-Fetch-Site header")
+		return errCrossOriginRequest
 	}
 
 	origin := req.Header.Get("Origin")
@@ -159,10 +159,15 @@ func (c *CrossOriginProtection) Check(req *Request) error {
 	if c.isRequestExempt(req) {
 		return nil
 	}
-	return errors.New("cross-origin request detected, and/or browser is out of date: " +
-		"Sec-Fetch-Site is missing, and Origin does not match Host")
+	return errCrossOriginRequestFromOldBrowser
 }
 
+var (
+	errCrossOriginRequest               = errors.New("cross-origin request detected from Sec-Fetch-Site header")
+	errCrossOriginRequestFromOldBrowser = errors.New("cross-origin request detected, and/or browser is out of date: " +
+		"Sec-Fetch-Site is missing, and Origin does not match Host")
+)
+
 // isRequestExempt checks the bypasses which require taking a lock, and should
 // be deferred until the last moment.
 func (c *CrossOriginProtection) isRequestExempt(req *Request) bool {